Agree with that;
> Actions have real life, often irreversible, effects on those around you
agree with that also;
> Many very dishonest people help themselves sleep at night by pretending that being polite excuses their actions
I could believe this is true, although I suspect most habitually dishonest people don't even see themselves as dishonest or think they're doing anything wrong.
None of these things are opposite to what the GP wrote though - so I don't understand what you're disagreeing with.
A classic example of the value of respect and courtesy in dealing with a difficult situation is when you have a problem with a food order at a restaurant. You can choose to shout at the person serving you, or approach the problem in a respectful and courteous way - in my experience, the latter is more effective at getting the best outcome, regardless of who is nominally at fault.
Respectful discourse also gives you an easy route to backing down when it turns out you've made the mistake, and some moral high ground if you need to escalate the situation.
Finally I would also say that even if someone is behaving unpleasantly towards you, that does not necessitate an unpleasant response. They may be having a terrible day for other reasons, and a little patience gives them a chance to relax a bit and respond in a more constructive way, which is to your benefit. It's hard for someone to stay angry with you when you're treating them fairly.
